In the LTE system, carrier aggregation (CA: Carrier Aggregation) for performing communication by simultaneously using a plurality of carriers is adopted, in which predetermined bandwidths are used as basic units (non-patent document 1). In carrier aggregation, a carrier which is a basic unit is called a component carrier (CC: component carrier).
When CA is performed, a PCell (Primary cell) that is a reliable cell for ensuring connectivity and an SCell (Secondary cell) that is an appendant cell are set for the user apparatus UE. The user apparatus UE connects to a PCell first, and then, an SCell can be added as necessary. The PCell is a cell similar to an independent cell for supporting RLM (Radio Link Monitoring) and SPS (Semi-Persistent Scheduling) and the like.
Addition and deletion of the SCell is performed by RRC (Radio Resource Control) signaling. Since an SCell is in a deactivated state right after it is set in the user apparatus UE, communication becomes available (scheduling becomes available) only by activating it.
In the user apparatus UE and a base station in the LTE system, HARQ (Hybrid ARQ) control is performed in an HARQ entity of a MAC (Media Access Control) layer (non-patent document 2). For example, in HARQ control for downlink data in the user apparatus UE, when decoding of downlink data (TB: transport block) succeeds, an ACK is returned to the base station eNB, and when decoding is failed, a NACK is returned to the base station eNB. An ACK/NACK (HARQ acknowledgements, transmittal acknowledgement information) is transmitted at a predetermined timing after downlink data reception (example: after 4 subframes) by a PUCCH (Physical Uplink Control Channel) set in a predetermined UL resource (non-patent document 3).